Nitrogen transformation and loss during co-composting of kitchen and garden wastes

Abstract: A large amount of kitchen and garden wastes is ever increasing, with the rapid development of urbanization in China. Kitchen waste is characterized by rich free fat content, high water and salt content. The inappropriate pore structure and organic composition can inhibit the organic degradation during aerobic composting, thereby leading to nitrogen loss in the form of ammonia volatilization. Garden waste is rich in cellulose, hemicellulose, lignin, and high-carbon substances, particularly hard to be degraded directly. Alternatively, a co-composting of kitchen and garden wastes can improve the physicochemical characters to make the mixed materials more suitable for composting. Therefore, the current work aims to investigate the co-composting feasibility of kitchen and garden wastes. The ratios of kitchen and garden wastes were set as 4∶1 (N1), 3∶1 (N2), and 2∶1 (N3) (dry weight basis). A 28-day experiment was conducted in the self-developed closed system of aerobic composting. The total material weight, the moisture content, and the aeration rate of composting mixtures were 2.5kg, 75%, and 0.2L/(kg·min), respectively. Some indexes were recorded during the process, including the fermentation temperature, pH, C/N, Germination Index (GI), NH3 and cumulative emissions, total N, organic N, ammonium N, and nitrate nitrogen. The specific rule was revealed to the nitrogen transformation and ammonia volatilization loss. The results showed that the temperature increased faster than other treatments, when the ratio of kitchen and garden waste was 2∶1 (N3), indicating the highest fermentation temperature (63.4℃). Meanwhile, the GI in N3 treatment exceeded 80% on the 21st day, meaning that the fermentation was significantly accelerated. The GI values of three treatments at the end of the process were 75.77%, 81.54%, and 84.50%, respectively. The products in the N2 and N3 treatment reached the standard of complete decomposing (>80%), while, those in the N1 only met the standard of basic decomposing (>60%). Therefore, a strong recommendation was given that the proportion of kitchen waste should not be higher than 80% of materials in the process of waste co-composting. The total nitrogen content decreased in the N1 and N2 treatment, whereas, it increased in the N3 fermentation. A high pH of products was obtained, due mainly to the fact that part of organic nitrogen was converted into ammonium nitrogen. The total nitrogen loss in N3 was the lowest at the end of composting, especially lower than that in the N1 and N2 by 30.30% and 12.96%, respectively. The nitrogen transformation demonstrated that the high fraction of garden waste reduced the NH3 emission and the loss of ionic nitrogen, thereby promoting the conversion of ammonium nitrogen to organic nitrogen and nitrate nitrogen, indicating a higher nitrogen content in compost products. An optimal ratio of kitchen waste to garden waste was 2∶1, indicating the treatment is feasible. The co-fermentation of kitchen and garden wastes can greatly contribute to the reduction of nitrogen loss. The finding can provide potential theoretical support to the co-composting for kitchen and garden wastes.